The present invention relates to a temperature-stabilized package that improves the performance of a chromatic dispersion compensation Bragg grating by providing a substantially uniform temperature around a fiber optic coil including the Bragg grating.
Telecommunication systems using optical fiber networks rely on a succession of short pulses of light to transmit coded information between widely separated locations. Light pulses include a range of wavelengths. It is well known that the velocity of light varies as a function of wavelength as the pulses of light move along an optical fiber. This phenomenon is known as chromatic dispersion. It occurs because longer wavelength light travels more slowly than those of shorter wavelength. As a pulse lengthens, it begins to interfere with the succeeding light pulse making it difficult to distinguish the end of one pulse from the beginning of the next. Typically, signals begin to merge as they travel through long optical fiber cables.
It is known to compensate chromatic dispersion using a Bragg grating to recompress the pulses to their original length. Light of shorter wavelength penetrates further into the grating than longer wavelengths, before reflection. The longer reflection path delays the shorter wavelengths to make them substantially coincident with the longer reflected wavelengths, and to compress all reflected wavelengths into the time interval of the originally transmitted pulse.
The reflection characteristics of a Bragg grating are known to change with temperature. It is preferable, therefore, to locate a dispersion compensating Bragg grating in a uniform temperature environment. The need for temperature uniformity in the vicinity of optical fiber coils has been addressed previously. U.S. Pat. No. 6,226,438, for example, describes a package for containment of an optical fiber that includes a Bragg grating. A housing, having a lower conductivity than a retaining member, provides containment of the optical fiber and the retaining member. The insulating material of the housing provides a primary defense against non-uniform heating of the package. Filler in the form of a gel provides improvement of thermal stability inside the housing. The need to maintain contact between substantially the entire length of the Bragg grating and a surface of the retaining member is a demanding requirement.
Other optical fiber devices subject to temperature-related output-drift include optical gyroscopes. Although different from fiber optic Bragg gratings, optical fiber gyroscopes operate best in a temperature-stabilized environment. For example, U.S. Pat. No. 4,702,599 describes a rotation-rate measuring instrument that uses an optical fiber coil. Fluctuations in ambient temperature induce measurement errors. Embedding the optical fiber in a conductive sealing compound and placing the sealed coil inside a housing constructed from a very good thermal conductor significantly reduces these errors. Thermal radiation striking the outer wall of the housing dissipates rapidly due to thermal conductivity of the housing. Redistribution of the heat provides a means for compensating temperature induced measurement errors. This reference teaches the need to place an optical fiber coil in intimate contact with a thermally conducting compound and thereafter enclose the sealed coil in contact with the inner wall of a double walled housing having an air gap between the walls. Heat bridges link walls of a relatively complex structure that comprises a material of high thermal conductivity.
A thermally stabilizing enclosure may include both thermally conducting and thermally insulating materials, as in U.S. Pat. No. 5,208,652. This describes an optical branching/coupling unit for an optical fiber gyroscope including a thermal buffer box that prevents the influence of temperature on gyroscope output. A heat transmitting case surrounds an optical fiber loop wound on a spool made from a material of high thermal conductivity. The heat transmitting case resides inside a heat insulating case contained inside a heat-transmitting casing that provides the outermost layer of the thermal buffer box. External changes in temperature are moderated during passage of heat through the alternating layers of heat conducting and heat insulating materials. As a result any temperature changes in the vicinity of the optical fiber coil are slight and uniform. The buffer box requires multiple alternating layers of thermally conducting and insulating materials.
Temperature compensation was attempted using only thermally insulating materials. In this case, U.S. Pat. No. 5,245,687 describes an optical fiber coil for a fiber optic gyro wound on a bobbin, contained in an annular case resting on a relatively massive support plate that is essentially a heat sink. The thermal conductivity of the bobbin and the case substantially equals that of the fiber coil. Both the coil and the case respond slowly to abrupt changes in ambient temperature to reduce drift in the output of a fiber optic gyro made from the coil. Such a construction teaches that the entire optical fiber coil is surrounded, essentially encapsulated, with a material of low thermal conductivity. Consequently, use of a low thermal conductivity bobbin suppresses the drift in the gyro output due to the influence of an ambient temperature change.
U.S. Pat. No. 5,416,585 describes a relatively complex approach for correcting fiber optic gyro drift rate error due to changes in temperature. Sensing of temperature differences between an optical fiber coil-carrying spool and a housing for the spool may be used to compensate drift rate error. Temperature differences, measured by sensors in the housing and/or the coil, provide input to associated electronic circuitry, connected to the sensors. The electronic circuit calculates the temperature difference between the gyro housing sensor and the coil spool sensor and produces and applies a correction factor to the output of the fiber optic gyro. The structure surrounding the coil in this case does not appear to provide a uniform temperature since any drift in gyro output, with time, requires detection and compensation by the external monitoring equipment that uses the electronic circuitry. Although not specified, there is indication that the housing is a metallic housing.
The previous discussion suggests the need for a relatively simple device for containment of optical fibers in a uniform temperature environment. Suitable devices should have few parts and be easy to assemble as packages that contain optical fibers, particularly fiber optic Bragg gratings, at a uniform temperature. This would allow a grating to operate, substantially without change, during exposure of a package to temperature gradients such as those present in enclosures that house power supplies and other heat generating components used for telecommunications networks.
The present invention satisfies the need for a simple, easily-assembled package that maintains a substantially uniform temperature inside a container for an optical fiber that preferably includes a long fiber optic Bragg grating. Construction of a container according to the present invention requires a material, such as copper or aluminum, having high thermal conductivity. Due to its long length the fiber optic Bragg grating may be coiled to fit inside the container. The package includes a housing coupled to the container of the fiber optic Bragg grating. While material selection is not necessarily limiting, preferably the housing according to the present invention comprises a material, such as a plastic resin, that is a poor conductor of heat. Use of the term coupled for attachment of the container to the housing indicates that intervening structures may exist between the two. Coupling means include those that minimize the temperature gradient across the high conductivity container, while placing the fiber optic Bragg grating in a region of substantially uniform temperature inside the container.
A preferred embodiment according to the present invention comprises a container coupled to the housing using a low area of contact between them. This reduces the amount of heat flowing from the housing to the container and influences the way in which regions of uniform temperature, referred to herein as isotherms, develop inside the container. Except at points of coupling, an air gap separates inner walls of the housing from the outer surface of the container. Minimal contact between housing and container provides additional thermal insulation.
Coupling of a container inside a housing preferably uses an interlocking hub structure, having mating parts formed integrally with either the outer surface of the container or the inner walls of the housing. To minimize heat transfer, the hub diameter may be essentially the minimum required to prevent separation or breakage of the coupling between the container and the housing. If one side of the housing is heated relative to the other, a temperature gradient will exist between opposite outer surfaces of the housing. Heat from outside the housing will reach the container primarily by way of the hub structure. Most heat will flow from a warmer hub towards the thermally conductive container, and radially outward from the warmer hub, through the rim of the container, then radially inward to a hub structure on the cooler side of the housing. This will create circular isotherms radiating outward from a hub and planar isotherms through the outer rim representing the thickness of the container. A fiber optic Bragg grating of a substantially flat optical fiber coil will lie in a uniform region of temperature corresponding to a circular isotherm, preferably close to the rim of the container. The temperature remains relatively constant along the Bragg grating whether or not a substantial portion of the grating actually touches the container.
An alternative embodiment according to the present invention dispenses with the hub structure. The inner walls of the housing may contact the outer surface of the container thereby losing the benefit of a thermally insulating air gap. Regions of uniform temperature in this embodiment have contours differing from the circular isotherms of the preferred embodiment previously described. In the absence of circular isotherms a circularly coiled optical fiber may not adopt a position corresponding to a uniform region of temperature, especially when there is a temperature gradient. Selection of conducting and relatively non-conducting material for the container and the housing produces a package that reduces the impact of thermal gradients, but the alternative embodiment is less effective for maintaining a uniform temperature over the length of the fiber optic Bragg grating. Additional layers of thermally conducting or insulating materials may be positioned outside the housing for further shielding against temperature gradients.
More particularly the present invention provides a temperature stabilization package comprising a hollow housing that includes at least one connecting element for coupling a container formed to include an internal cavity. The connecting element holds the container and the hollow housing in a substantially spaced-apart relationship. Also, the internal cavity has a substantially uniform region of temperature therein, protected from unstable thermal conditions outside the temperature stabilization package. The housing further includes a floor having a separating wall formed integrally therewith to retain the substantially spaced-apart relationship between the container and the housing.
In another embodiment, a temperature stabilization package according to the present invention includes a hollow external housing comprising a thermally insulating material and a container formed to include an internal cavity using a thermally conducting material. The container resides inside the hollow external housing, and the internal cavity maintains a substantially uniform region of temperature, shielded from unstable thermal conditions outside the hollow external housing of the temperature stabilization package.
A preferred temperature stabilization package comprises a hollow external housing comprising a material having low thermal conductivity. The hollow external housing includes a holder and a lid to cover the holder that includes a floor having at least one projection and a separating wall around the projection. Also, the lid includes an internal surface having at least one post formed therein. A container, included in the temperature stabilization package, has a base and a cover over the base to form an internal cavity of the container. The base has at least one lower socket on an outer face thereof, and the cover includes at least one upper collar. Coupling of the container inside the hollow external housing results from engaging the at least one lower socket with the at least one projection and the at least one upper collar with the at least one post. This holds the container substantially surrounded by, but spaced from, the separating wall and the hollow external housing. The internal cavity maintains a substantially uniform region of temperature therein, shielded from unstable thermal conditions outside the hollow external housing of the temperature stabilization package.
The present invention further provides a temperature stabilized chromatic dispersion compensation module comprising a hollow external housing comprising a thermally insulating material, and optionally including at least one connecting element. A container formed to include an internal cavity, using a thermally conducting material, fits inside the hollow external housing coupled to the at least one connecting element (when present). This holds the container and the hollow external housing in a substantially spaced-apart relationship. The internal cavity maintains a substantially uniform region of temperature therein, shielded from unstable thermal conditions outside the hollow external housing of the temperature stabilization package. A fiber optic coil having a Bragg grating therein positioned in the uniform region of temperature inside the internal cavity of the container provides chromatic dispersion compensation of light passing through the fiber optic coil.
The output of a fiber optic Bragg grating may be stabilized according to the present invention using a method that comprises the steps of initially providing a temperature stabilization package comprising a hollow housing including at least one connecting element, and a container including an internal cavity. Coupling of the container to the housing uses the at least one connecting element to hold the container and the hollow housing in a substantially spaced-apart relationship. The internal cavity has a substantially uniform region of temperature therein, protected from unstable thermal conditions outside the temperature stabilization package. Placing a fiber optic Bragg grating in the substantially uniform region of temperature stabilizes the grating against temperature-related output variation.
Definitions
The following definitions provide clarification of terms used herein.
A xe2x80x9chousingxe2x80x9d may be combined with the terms xe2x80x9chollowxe2x80x9d or xe2x80x9cexternalxe2x80x9d or other similar adjectives to describe an enclosure formed from a thermally insulating material, i.e. having low thermal conductivity, by any of a number of known forming methods. Preferably the housing comprises a plastic resin formed by conventional molding techniques including thermoforming and injection molding.
A xe2x80x9ccontainerxe2x80x9d as used herein is preferably thermally conductive and formed using a metal such as copper or aluminum or similar metals or metal alloys having a thermal conductivity of more than 100 W/m-K. Common metal forming methods such as casting, machining and stamping may be used to provide a container according to the present invention.
A xe2x80x9cseparating wallxe2x80x9d refers to a structure inside a housing, preferably integrally formed therewith, to aid in maintaining a spaced-apart relationship between a container and housing of a temperature stabilized optical fiber package according to the present invention.
The term xe2x80x9caccess portxe2x80x9d refers to a passage through a housing to allow terminal ends from a Bragg grating, residing in a cavity in a container, to extend from the housing for connection to optical components outside the housing. As an alternative, a fiber optic connector may be positioned in an access port to facilitate connection of the Bragg grating to other optical components.
A xe2x80x9cconnecting elementxe2x80x9d means any one of a variety of permanent or releasable connecting structures used to hold a container inside a housing to couple the two in a relatively fixed, preferably spaced-apart relationship. Suitable connecting elements include rivets, studs, projections, posts, sockets, collars, mechanical fasteners, and interference fasteners such as hook and loop fasteners and similar forms of releasable connectors.
The beneficial effects described above apply generally to the exemplary devices and mechanisms disclosed herein of the temperature stabilization packages for optical fibers. The specific structures through which these benefits are delivered will be described in detail hereinbelow.